Agreed, it's hard to say for sure. Another possibility is that he may have a paraphilia/fetish where someone becomes aroused by things that are not considered to be "normal." These things are not necessarily bad unless they become overwhelming and stop the person from being functional or that it hurts another person. Some people like shoes, some like furry outfits, it could be any number of things. If it's not debilitating or harmful, perhaps there is nothing to worry about. You did say he felt ashamed though. I think he will have to decide if he wants to decrease the behavior or accept it and control it so that he is no longer ashamed.